Digital Subscriber Line

The biggest rival to the cable modem in the broadband Internet business is the digital subscriber line (DSL). DSL, like its predecessor ISDN, appeals to the telephone companies who might be able to use the existing POTS analog wiring to provide high-speed Internet access.

Not every type of DSL is suitable for existing wiring; however, all but the fastest, most expensive types can sometimes be used with the existing POTS plant. DSL is also appealing to businesses that don't have access to cable modems but are looking for a high-performance, lower-cost alternative to the expensive ISDN services that top out at 128Kbps.

One advantage of DSL compared to its most popular rival—cable modems—is that cable modems share common bandwidth, which means that a lot of simultaneous use by your neighbors can slow down your connection. If you use DSL you don't have this concern; whatever bandwidth speed you pay for is yours—period.

How DSL Works

DSL takes advantage of the broadband nature of the telephone system, using the system's capability to carry signals at multiple frequencies to allow both high-speed Internet traffic and phone calls at the same time. Two methods for sending and receiving signals are used by the most common type of DSL, Asymmetric DSL (ADSL):

  • Carrierless Amplitude/Phase (CAP)

  • Discrete Multitone (DMT)

Most early DSL installations used CAP, which splits the telephone line into three frequency bands. Exact frequency usage varies by system, but most typically, the divisions resemble the following:

  • Voice calls use frequencies from 30Hz to 4KHz. This frequency is also used by answering machines, fax machines, and alarm systems.

  • Upstream data such as Web page requests and sent email uses frequencies between 25Hz and 160Hz.

  • Downstream data such as received Web pages and email uses frequencies between 240KHz and 1.5MHz.

Some systems use the 300Hz–700Hz range for downstream data and frequencies of 1MHz and above for upstream data. Because voice, downstream, and upstream data use different frequencies, you can talk, surf, and send email at the same time.

DMT, the system used by most recent ADSL installations, divides the telephone line into 247 channels that are 4KHz wide. If a particular channel has problems, a different channel with better signal quality is used automatically. Unlike CAP, DMT uses some channels starting at around 8KHz to send and receive information.

Both types of signaling can have problems with interference from telephones and similar devices, so devices called low-pass filters are used to prevent telephone signals from interfering with signals above the 4KHz range, where DSL signals begin. The location of these filters depends on the type of DSL you use and whether you are installing DSL service yourself.

At the central switch, DSL data is transferred to a device called a DSL access multiplexer (DSLAM), which transfers outgoing signals to the Internet and sends incoming signals to the correct DSL transceiver (the correct name for the so-called "DSL modem" that connects to your computer).

Who Can Use and Who Can't

DSL services are slowly rolling out across the country, first to major cities and then to smaller cities and towns. As with 56Kbps modems, rural and small-town users are probably out of luck and should consider satellite-based or fixed wireless Internet services where available for a faster-than-56Kbps experience.

Just as distance to a telephone company's central switch (CS) is an important consideration for people purchasing an ISDN connection, distance also affects who can use DSL in the markets offering it. For example, most DSL service types require that you be within about 18,000 feet (about 3 miles) wire distance to a telco offering DSL; some won't offer it if you're beyond 15,000 feet wire distance because the speed drops significantly at longer distances.

Repeaters or a local loop that has been extended by the telco with fiber-optic line might provide longer distances. The speed of your DSL connection varies with distance: The closer you are to the telco, the faster your DSL access is. Many telcos that offer some type of DSL service provide Web sites that help you determine whether, and what type of, DSL is available to you.

If you want to locate DSL service providers in your area, compare rates, and see reviews from users of the hundreds of ISPs now providing DSL service, set your browser to DSL Reports. The site provides a verdict on many of the ISPs reviewed, summarizing users' experiences and ranking each ISP in five categories.

Even if your telco's central switch is well within wire distance range of your location, that's no guarantee that you qualify for DSL service. The design and condition of the wiring plant connecting your location with the central switch can prevent you from qualifying for DSL service.

Because DSL service depends on successful sending and receiving of high-frequency data, a telephone wiring plant that blocks high-frequency signals can't be used for DSL service. Some of the typical issues with telephone lines that aren't DSL-friendly include:

  • Loading coils. These amplifiers boost voice signals and are sometimes called voice coils. Unfortunately, these block the high-frequency signals needed by DSL service.

  • Bridge taps. Used to extend service to new customers without running separate lines all the way back to the central switch. Bridge taps can create a circuit that's too long for DSL service.

  • Fiber-optic cables. Used to carry a lot of signals in a small physical space, fiber-optic cables use analog-to-digital (A/D) and digital-to-analog (D/A) converters where they connect to copper telephone lines. A/D and D/A converters can't pass DSL signals through to their destinations.